Enhancement: Inform about successful retries after errors

When a recoverable error is encountered, restic shows a warning message saying
that it's retrying, e.g.:

`Save(<data/956b9ced99>) returned error, retrying after 357.131936ms: ...`

This message can be confusing in that it never clearly states whether the retry
is successful or not. This has now been fixed such that restic follows up with
a message confirming a successful retry, e.g.:

`Save(<data/956b9ced99>) operation successful after 1 retries`

https://github.com/restic/restic/issues/1734
https://github.com/restic/restic/pull/2661
